Transcript: Cepheid variables are luminous stars with variations in a range of periods of one to fifty days. The physics of their pulsation is well understood, and empirically for stars with well measured distance by parallax, there’s a well determined relationship between the period of the pulsation and the luminosity of the star. More luminous Cepheids have longer periods. Astronomers therefore isolate Cepheids in a distant cluster by taking images over a period of several months to identify the variable stars and measure their periods. The period then leads to a prediction of the luminosity. That is combined with the apparent brightness to yield a distance. The Cepheid distance measurement technique is among the most accurate in astronomy with a precision of ten percent.